Motion compensated transform coding has been widely adopted in various coding standards, where block transform is applied to motion-compensated residues. The motion compensated inter-frame coding system also uses intra-frame mode periodically or adaptively. During the coding process, transform coefficients are quantized in order to reduce bitrate and consequently artifacts are introduced. The artifacts are more visible at boundaries around transform blocks. In order to alleviate the coding artifacts, a technique called deblocking has been developed which applies filtering across block boundaries adaptively. The deblocking technique is also called deblocking filter in the field of video coding.
The deblocking process applies filters to boundary pixels and the filter coefficients are related to boundary strength of associated boundary. The deblocking filter process performs a series of testing to determine the boundary strength of a selected boundary. According to the derived boundary strength, filter ON/OFF decision is made. Furthermore, when an ON decision is made, filter coefficients are selected according to boundary strength. However, the conventional filter decision process contains redundancy, which unnecessarily consumes system computational resource. Furthermore, the redundancy may also degrade system performance in terms of compression efficiency. It is desirable to remove the redundancy in order to conserve system resources and/or to improve system performance.